(a) Field of the Invention
The present invention relates generally to a method and apparatus for initializing a Global Positioning System (GPS) receiver for use in a communication network, and more particularly, to a method and apparatus for initializing a GPS receiver forming part of a communication network in such a manner that facilitates rapid determination of the position of the GPS receiver upon startup of the GPS receiver.
(b) Description of Related Art
Typically, a GPS receiver relies on GPS satellite trajectory parameters stored in memory during recent operation, a time estimate from a running clock or user input, and a position estimate from memory or user input to perform a startup faster than a xe2x80x9ccold startxe2x80x9d. If any of this information is missing, a cold start will be necessary and the time to first fix (TTFF) may be 1-3 minutes.
In general, one common strategy for a GPS receiver integrated into a communication network is to either continually track GPS satellites, or cycle on at timed intervals to obtain a fix, resynchronize a local clock, and download GPS parameters. The disadvantages to this approach are: 1) longer acquisition times in general; 2) larger power consumption and processing drain; and 3) the need for GPS acquisition at times when terminal is not otherwise in use (in the case of a handheld terminal, this is particularly problematic; when the terminal is not in use it may be stowed somewhere, such as a pocket or briefcase, where GPS satellite visibility is very poor).
Another strategy involves a wholly integrated approach, where a terminal reports intermediate GPS measurements to the network, and the network performs the actual positioning computations. The disadvantages of this approach are 1) increased data transfer from the terminal to the network is needed; 2) complex network computing facilities are required to handle each terminal separately; and 3) the terminal is unable to perform GPS positioning when network is not available.
There are technologies emerging in communication networks, primarily for Emergency-911 systems, such as those developed by Navsys, Inc. and Snaptrack, Inc., that use wholly integrated approaches to determination of terminal positions, meaning that the position determination requires substantial handshaking and cooperation between remote terminals and the network infrastructure. (As used herein, the term xe2x80x9cterminalxe2x80x9d describes a mobile unit within a communication network, such as a cellular phone.)
The invention is directed to a comprehensive method of utilizing a communications network (in this particular design, it is a GEM (Geo-Mobile) satellite communication network system) to initialize a Global Positioning System (GPS) receiver (such as one contained within a handheld user terminal) to enable the GPS receiver to quickly acquire GPS satellite signals and perform positioning calculations. By systematically feeding the GPS receiver key pieces of information such as, for example, (GPS satellite trajectories, time estimate, position estimate, and additional positional references), the time to first fix (TTFF) of the GPS receiver can be significantly reduced. One novel aspect of the invention is the series of actions taken by the GEM system to determine and provide, to the GPS receiver, information that otherwise may not be available.
The present invention is useful for a Geo-mobile (GEM) satellite phone network, where any and all terminals may be initialized at any time by the generic broadcast of the GEM satellite, but could be applied to a broad class of communication networks. A network may require that a terminal (e.g., a wireless telephone) determine and report its position before each call it places with the network. For this reason, fast GPS-based positioning capability within each terminal is required. This invention provides the capability to automatically and simultaneously initialize all terminals with the information required to obtain minimal GPS acquisition times.
It has been determined that minimal position determination times are achievable if the following five pieces of information are available to a terminal, and hence to its integrated GPS receiver:
1. Satellite parameters describing orbital trajectories for all GPS satellites in view to the terminal, used by the terminal to compute satellite positions at a given instant in time (a critical step in the receiver""s computation of its own position);
2. GPS time estimate to within a few milliseconds;
3. A rough position estimate to within a few hundred kilometers;
4. A GPS almanac; and
5. Additional positional references.
As will be appreciated by those skilled in the art, providing orbital trajectories for all GPS satellites visible to the terminal is optimal. However, the invention contemplates utilizing orbital trajectories for fewer than all visible GPS satellites as well.
The system in accordance with the invention continuously determines and provides all five of the above pieces of information to all terminals in the coverage area, so that any terminal, at any time, is able to perform a fast position determination.
The GEM system, for example, is designed to provide these key pieces of information as follows:
1. GPS satellite trajectories; Each ground station supports a continually active GPS receiver that tracks all visible GPS satellites, and stores satellite parameters precisely describing their orbits, for all of them. The ground station predicts which GPS satellites are in view to each distinct service area (spot beam), and the GEM satellite broadcasts local trajectory coefficients (computed at the ground station) for those GPS satellites through each spot beam.
2. GPS time: The ground station is synchronized to GPS time, via the ground station""s active GPS receiver. GPS time, adjusted for propagation delay to within a few milliseconds, is broadcast with the satellite information.
3. Rough position estimate: The terminal measures relative signal strengths of broadcast channels in neighboring spot beams. Based on these measurements, a position estimate typically accurate to about 100 km can be computed, although a position estimate accuracy up to about 1,000 km may be acceptable.
4. A GPS almanac is downloaded at the ground station and re-broadcast to the coverage area.
5. With further network initialization, GPS acquisition may be possible in poor GPS satellite visibility situations. An extremely accurate time reference (e.g., accurate to within two microseconds) could reduce the number of required GPS satellite signal acquisitions from four to three, provided some degradation in fix accuracy is acceptable. If the communication network can provide precise pieces of position information (such as altitude, and/or distance from a known reference), the number of GPS satellites needed may be further reduced.
In accordance with one aspect of the present invention, a method is provided for initializing a GPS receiver to rapidly acquire GPS satellite signals for establishing a precise estimate of the position of the GPS receiver. The method comprises the steps of: broadcasting a signal representative of orbital trajectories of one or more GPS satellites within view of the GPS receiver; broadcasting a time synchronization signal; calculating a rough estimate of the position of the GPS receiver may be calculated; and inserting the signal representative of orbital trajectories, the time synchronization signal, and the rough estimate of the position of the GPS receiver into the GPS receiver.
Preferably, the time synchronization signal is accurate to within about five milliseconds and the rough estimate of the position of the GPS receiver is accurate to within about 1,000 kilometers.
Preferably, the orbital trajectories signal is broadcast via a satellite, using an idle communication channel. Also preferably, the time synchronization signal is broadcast via a satellite, using an idle communication channel.
Obviously, it is desirable to have a time synchronization signal that is extremely accurate (e.g., to within a few microseconds). However, such accuracy levels are generally not feasible.
In accordance with another aspect of the present invention, a communication network comprises at least one communication station, a plurality of GPS satellites, and a terminal. The communication station includes capability for sending communication signals, GPS satellite trajectory signals, and time synchronization signals to the terminal. The terminal includes apparatus for receiving communication signals from the communication station, apparatus for sending communication signals to the communication station, apparatus for receiving GPS signals from the GPS satellites, and apparatus for processing the GPS satellite trajectory signals and time synchronization signals to rapidly determine the position of the terminal.
In accordance with yet another aspect of the invention, a method of initializing a remote GPS receiver to rapidly acquire GPS satellite signals for establishing a precise estimate of the position of the remote GPS receiver is provided. The method comprises the steps of: providing an active GPS receiver at a fixed location which computes precise GPS satellite trajectories; broadcasting a signal representative of the precise GPS satellite trajectories of one or more GPS satellites within view of the remote GPS receiver; broadcasting a time synchronization signal; calculating a rough estimate of the position of the remote GPS receiver; and inserting the signal representative of orbital trajectories, the time synchronization signal, and the rough estimate of the position of the remote GPS receiver into the remote GPS receiver.
In accordance with still another aspect of the invention, a terminal for use in a communication network is provided. The communication network comprises at least one communication station and a plurality of GPS satellites. The communication station includes apparatus for sending communication signals, GPS satellite trajectory signals, and time synchronization signals to the terminal. The terminal comprises an apparatus for receiving communication signals from the communication station, an apparatus for sending communication signals to the communication station, an apparatus for receiving GPS signals from the GPS satellites, and an apparatus for processing the GPS satellite trajectory signals and time synchronization signals to rapidly determine the position of the terminal.
In accordance with yet another aspect of the invention, a gateway station for use in a communication network is provided. The communication network comprises at least one terminal, a communications satellite and a plurality of GPS satellites. The gateway station comprises a continually-tracking GPS receiver, apparatus for computing GPS satellite trajectory data for GPS satellites visible to each terminal, apparatus for computing a rough position estimate for each terminal, and apparatus for sending signals representing GPS satellite trajectory data, time synchronization signals, and signals representing rough position estimates to each terminal.
The invention disclosed herein has the distinct advantage that the terminals possess self-contained fully functional GPS receivers. The invention acts as a supplement to greatly accelerate GPS receiver operation, not replace it. Thus, each terminal has full GPS functionality even when the communication network is not available. In addition, the handshaking required to compute a terminal position is reduced to a 1-way generic broadcast from the communication network to the terminals. All terminals in a given geographic cluster of up to a few hundred km receive the same information and proceed with the accelerated GPS acquisitions. No information is required to be transmitted from the terminal to the network. The position, when computed, is available for immediate display to the user of the terminal, and may be transmitted to the network if desired.
By using the invention, the GEM system will automatically provide the GPS receiver with enough information to optimize startup, regardless of what is in memory, without a running clock, and with no user intervention. The result is a more consistent, and much faster initial position fix, that typically takes only a few seconds.
The invention provides enhanced customer satisfaction and power savings in the terminal, which is especially important where the terminal is a portable handset having a finite battery charge. It is, further, particularly effective whenever a user tries to place a phone call in a network that requires a GPS position fix before a call is allowed. GPS positioning functionality consistently delaying call setup for 1-3 minutes would have a devastating effect on product quality. With use of the present invention, the GPS process should delay registration or call setup no more than a few seconds. The second benefit of the invention is that once a GPS position is obtained, the GPS receiver may be turned off. A GPS receiver may consume up to about 0.5 W while on, and use of the invention would greatly reduce the amount of on-time, and therefore power consumption.
The invention itself, together with further objects and attendant advantages, will be best understood by reference to the following detailed description, taken in conjunction with the accompanying drawing.